1. Field of the Invention
The subject invention relates to fabrics, in general, and to a reversible single layer forming fabric having a preponderance of dominating cross-machine direction floats on each face, in particular.
2. Description of the Prior Art
In papermaking machines, paper stock, also called furnish or stuff, is fed onto the top surface or outer face of traveling endless papermaking belts, which serve as the papermaking surface of the machine. The bottom surface, or innerface, of the endless belts is supported on and driven by rolls associated with the machine. Papermaking belts, also known as Fourdrinier wires, forming media or forming fabrics, are commonly configured from a length of woven fabric with its ends joined together in a seam to provide an endless belt. The fabric may also be constructed by employing an endless weave process, thereby eliminating the seam. Either fabric generally comprises a plurality of machine direction yarns and a plurality of cross-machine direction yarns which have been woven together on a suitable loom.
Initially, forming fabrics were woven wire structures made from materials such as phosphor bronze, bronze, stainless steel, brass, or suitable combinations thereof. Recently, in the papermaking field, it has been found that synthetic materials may be used, in whole or in part, to produce forming fabrics of superior quality. Today, almost all forming fabrics are made from polyesters such as Dacron or Trevira, acrylic fibers such as Orlon, Dynel and Acrilan, copolymers such as Saran, or polyamides such as Nylon. The warp and weft yarns of the forming fabric may be of the same or different constituent material and/or constructions, and may be in the form of a monofilament or multifilament yarn.
In the prior art, various single layer forming fabrics have been made by employing weave patterns ranging from the 1.times.1 plain weave, progressing in stages to the 4.times.1 straight and broken twill or satin weaves. Each of these fabrics suffer from the same shortcoming: the rapid wear of the knuckles contained on the innerface of the fabric when the knuckles are brought into contact with the rolls during the papermaking process. As used herein, a knuckle is a portion of a weft yarn that passes over (or under) only one warp yarn, or a portion of a warp yarn that passes over (or under) only one weft yarn before interweaving.
Many of the prior art forming fabrics employ cross-machine direction floats on either the outer or the inner surface. As used herein, a float is a portion of a weft yarn that passes over (or under) two or more warp yarns, or a portion of a warp yarn that passes over (or under) two or more weft yarns before interweaving. For example, a weft yarn that passes over three warp yarns before interweaving will, herein, be referred to as a three-float. These cross-machine direction floats, in conjunction with the knuckles present on the inner surface of the fabric, have a tendency to curl the edges of the fabric in a direction towards the outer surface on which the cross-machine direction floats are located. It is believed that such edge curling results from tension differentials on the top and bottom surfaces of the fabric, caused by differences in the area occupied by knuckles and floats on one surface of the fabric and knuckles and floats on the other surface.
Further, the relationship between knuckle and float placement on prior art fabrics leads to a trade-off between the smoothness of the finished paper and the life of the fabric due to machine wear. For example, if the floats are on the paper-receiving surface, a smoother paper surface will result with an attendant decrease in fabric life due to knuckle wear. Alternatively, if the floats are on the machine-contacting surface, fabric life will be increased due to the greater wearing surface provided by the floats, but the smoothness of the paper will be greatly diminished because of the preponderance of knuckles occupying the paper-receiving surface.
If the preponderance of cross-machine direction floats are on the outer or paper-receiving surface of the fabric, the edges have a tendency to curl upwardly and, hence, cause the paper stock to "roll" down the curled edge to thereby produce a raised portion on the finished paper. If the cross-machine direction floats are on the inner or roll-contacting surface of the fabric, the edges tend to curl downwardly, and there is a tendency for the fabric edges to wear down as they contact the rolls, suction boxes, and various other support equipment of the papermaking machine. Such curling tendency also results in grooves being worn into the rolls, suction boxes, etc. Thus, with upward edge curl, the full width of the forming fabric cannot be used to make paper, and with downward edge curl, the papermaking machine is, itself, subjected to excessive wear.
In the prior art, for the most part, there is also a trade-off between factors, such as, strength and stability on the one hand and permeability on the other. For example, if strength and stability were to be maximized, as by using more yarns per inch in the finished fabric, then the permeability of the fabric would fall correspondingly. Therefore, because forming fabrics require high permeabilities to ensure adequate drainage, strength and stability are often sacrificed in favor of drainage considerations.
It is, therefore, desirable to produce a forming fabric in which edge curl and knuckle wear are kept to a minimum without sacrificing the quality of the finished paper. It is also desirable to produce a forming fabric having strength and stability, and yet also having high permeability. The present invention is directed toward producing such a fabric.